Technical development is currently in progress relative to recording/reproducing apparatus using an optical tape as a recording medium and such apparatus is schematically shown in FIG. 1. Typically, a pair of rotary scanning optical head devices are employed in such optical tape recording/reproducing apparatus, for example, having two laser beam sources and two heads disposed opposite to each other at an angular interval of 180 degrees. More specifically, in the optical head device of FIG. 1, two laser diodes 1A, 1B are incorporated in a rotary drum D at symmetrical positions so as to be spaced apart from each other at a predetermined interval, and a reflection prism 2 is interposed between laser diodes 1A, 1B. Along the light paths of the laser beams directed by reflection prism 2 there are sequentially disposed collimation lenses 3A, 3B, grating plates 4A, 4B, polarized beam splitters 5A, 5B, quarter-wave plates 6A, 6B, and objective lenses 7A, 7B. Recording/reproducing laser beams emitted from laser diodes 1A, 1B are forwarded via the above-described optical systems to the outside of the rotary drum D through apertures 8A, 8B, respectively, to finally arrive at an optical tape T that is helically arranged around rotary drum D over an angular range of substantially 180 degrees. In this manner a signal is recorded on the optical tape T, or a recorded signal on the tape T is reproduced therefrom, as slanted tracks of data running transverse to the length of the record medium.
The laser beam reflected from the optical tape T that can serve as either a reproduced data signal or a focusing/tracking control signal is passed via objective lenses 7A, 7B, quarter-wave plates 6A, 6B, and polarized beam splitters 5A, 5B to reach light receiving lenses 9A, 9B, whereby it is further irradiated onto two photo detectors 10A, 10B, respectively. The output signals of detectors 10A, 10B are supplied to reproduced-signal processing circuits and servo circuits (not shown).
The focusing/tracking control signals produced by such servo circuits are supplied to servo controlled actuators 11A, 11B, which may comprise dual-axis lens moving mechanisms or the like, so that the objective lenses 7A, 7B are driven to correct focusing positions or to correct tracking positions.
For example, when a pair of optical heads are incorporated in a rotary drum as described above, the optical heads sequentially scan the optical tape every half rotation of the rotary drum in compliance with the motion of the optical tape, thereby performing a helical-scan recording/reproducing operation.
In the case where the tape winding angle is 180 degrees as in the above-described example, each optical head comes to have in accordance with the rotation of the rotary drum D as illustrated in FIG. 1, a scanning period, hereinafter referred to as A-segment, facing toward optical tape T and a non-scanning period, hereinafter referred to as B-segment, facing away from optical tape T.
A reflected laser beam is not obtained during the non-scanning period of the head, so that the tracking and focusing control signals produced on the basis of the reflected laser beam are not obtained until the optical head advances into the A-segment again. Therefore, the focusing or tracking condition upon entering from the B-segment into the A-segment is rendered unstable because there is no servo control. Moreover, even upon detection of a focusing error signal or a tracking error signal after entering into the A-segment, it is still impossible to achieve rapid control for moving the laser beam spot to a desired track or placing the same in a correct focus condition in response to a detected error signal. Furthermore, as the control error signal becomes greater, a longer time is required for pulling the lens into correct focus or into the correct tracking position due to transient vibrations.
When such required servo pull-in time interval is prolonged, an effective time interval that equals the A-segment minus the pull-in time interval, which is the effective recording/reproducing time, is shortened to raise consequently a serious problem with respect to the record signal transfer rate or the recording density. In order to solve this problem, it has been customary heretofore that the tracking and focusing control errors at the end of the A-segment are held as in the preceding stage, and the position of the actuator is locked until the next entering of the optical head into the A-segment.
Nevertheless, the spacing Z between tape T and the optical head in the A-segment during the motion of tape T is generally affected, as illustrated in FIG. 2(a), by a thin air film layer or space formed due to flow of air from a point A.sub.s in FIG. 1 where the moving tape T is initially brought into contact with drum D, so that the distance Z between tape T and drum D at the beginning of the A-segment is gradually reduced in accordance with the rotation, until finally tape T is brought into its closest contact with drum D at the end of the A-segment.
By means of the focusing control signal that follows up such change in the distance Z in the A-segment, the absolute position of objective lens 7 is changed in the effective time interval, as shown in FIG. 2(b). If there occurs a considerable difference in the correct focusing position between the start portion and the end portion of the A-segment, and the focusing actuator is controlled during the non-scanning period to hold the lens position at the end of the A-segment, then the optical head enters into the next A-segment at a position considerably spaced apart from the correct focusing position. Therefore, at the time of pulling in to the correct focusing position, as illustrated, stable servo control cannot be rapidly executed and effective reduction of the pull-in time interval is presented.
The above problems apply to tracking control as well. For example, due to some long-cycle meander of the tape path or the tape edges, the track position is prone to have an S-shaped deviation as shown in FIG. 3(a). Therefore, the tracking control signal will have a waveform as shown in FIG. 3(b), which eventually necessitates a relatively long pull-in time interval.